1. Field of the Invention
The present invention relates to a brushless d.c. motor, and more particularly to an inverter driving circuit for brushless d.c. motor adapted to pass Pulse Width Modulated signal to switching elements of the inverter.
2. Description of the Prior Art
In general, a brushless d.c. motor is more and more used in fields where dangerous spark or arc phenomena is not supposed to be generated due to motor brushes or commutators. Also, the motor is often employed in an application such as in a hard disk where a constant speed operation is absolutely needed.
The brushless d.c. motor has a permanent magnet rotor and a stator armature winding excited by means of electrical switching. Rotation of the armature magnet field is accomplished using power transistors acting to change a direction of electric current flowing into the armature windings.
In order to synchronize the permanent magnet field and the rotation magnet field by the armature of the stator, a positional sensor is installed around a shaft of the rotor to ensure to be switched at an appropriate time.
Rotor position may be detected by sensing a voltage induced in the stator armature windings instead of using the position sensor. Driver for brushless d.c. motor designed based upon this principle consists of, as shown in FIG. 1, a filtering unit 1, a rectifying unit 2, a capacitor C, an inverter 3, a rotor-position detecting unit 5 for a brushless d.c. motor 4, a control unit 6, an inverter driving unit 7, and a power supply unit 8.
The filtering unit 1 serves to remove noise components embedded in A.C. power of 220 V, 60 Hz fed from the external and outputs the filtered A.C. power to the rectifying unit 2 which rectifies the A.C. power to produce a corresponding D.C. power needed in an inverter 3.
The inverter 3 consists of a plurality of switching elements Q1 to Q6, and a plurality of diodes D1 to D5, and switches the elements in response to the switching element driving signal input from the inverter driving unit 7, to supply 3-phase power of U, V and V phases to the brushless d.c. motor 4.
The rotor-position detecting unit 5 senses a voltage induced in each phase coil of the brushless d.c. motor for detecting the rotor position.
The control unit 6 is operated to switch the respective switching elements Q1 to Q6 under a user's manipulation to control the start-up, operation and speed of the motor. For these purposes, the control unit 6 produces switching element driving signals for use in switching the respective switching elements Q1 to Q6 in response to outputs from the rotor-position detecting unit 5 and then outputs them to the inverter driving unit 7.
The inverter driving portion 7 passes the switching element driving signal output from the control unit 6 to the respective switching elements Q1 to Q6 to switch those elements.
The power supply unit 8 transforms and rectifies the A.C. power input through the filtering unit 1 and then distributes the resultant power into the control unit 6 and the inverter driving unit 7 as the driving power VDC1 to VDC3.
The detailed circuit configuration of the inverter driving unit 7 is shown in FIG. 2, which is illustrative of the driving of U phase-related switching elements, but their operational principle is equally applicable to the other V and W phases-related switching elements.
The inverter driving unit 7 is constructed as illustrated in FIG. 2, which is only illustrative of the driving of U phase-related switching elements Q1, Q2, but their operational principle is equally applicable to the other V and W phases-related switching elements.
More detailed, the inverter driving portion 7 in FIG. 2 is comprised of a general-purpose photo-coupler PC1 and a high speed switching photo-coupler PC2. The general-purpose photo-coupler PC1 serves to pass, with an internal photo-coupling thereof, the switching element driving signal of low frequency from the control unit 6 to a base terminal of the first switching element Q1(or, PNP-type power transistor) in order to switch the element Q1 for a phase change.
The high speed switching photo-coupler PC2 receives the switching element driving signal of high frequency, or pulse width modulated signal from the control unit 6, in which the receipt of the signal can be made through an internal photo-coupling thereof, after shaping it, and then passes it to a base terminal of the second switching element Q2. This causes the second switching element Q2(or, NPN-type power transistor) to be switched, resulting in the supply of the power source to the stator armature windings of the brushless d.c. motor.
The power supply unit 8 provides various power source voltages DCV1, DVC2 and DVC3 to the control unit 6, general-purpose photo-coupler PC1 and high speed switching photo-coupler PC2, respectively, for their activation.
Reference symbols R1 to R4 and D1, D2 in the drawings denote resistors and diodes, respectively.
For the forgoing brushless d.c. motor, either an upper or lower switching elements in the inverter 3 may be switched at high speed by the PWM signal supplied thereto as a driving signal so as to result in the supply of the power source to the motor, wherein a pulse width of the PWM signal is varied for the control of the start-up and speed of the motor.
The PWM signal separated from the power source for the motor needs to use a photo-coupler for passing a signal to the proper switching element. The low frequency signal for driving a switching element, namely for a phase change is possibly passed to the relevant switching element via the general-purpose photo-coupler PC1, but a high speed switching photo-coupler has to be employed for preventing the distortion of the signal which is passed through the high speed switching photo-coupler. This is because a high frequency signal of the order of 20 KHz is used for the purpose of avoiding an audible noise.
The prior art inverter needs a plurality of high speed switching photo-couples assigned for each phase, but these photo-couplers cost high. This makes it difficult to adopt the high speed switching photo-coupler in the brushless d.c. motor.